In mining and construction operations, tools are used which have a rotatable head carrying a plurality of pick type cutters or bits. In a construction tool for a planing operation, for example, the head takes the form of a drum and is rotatably driven and carries a plurality of pick type bits disposed in a helical pattern on the surface thereof. For supporting each bit, the head carries a mounting or support block having a socket therein for retaining the bit. The bit has a cylindrical shank disposed in the socket and has a conical head extending beyond the block. A pointed cutting tip of wear resistant material such as cemented tungsten carbide is mounted on the outer end of the head. Each bit is rotatably supported in the support block and, due to inclination of the bits on the head, the bits are adapted to rotate during operation for the purpose of maintaining the tip in a pointed condition throughout the operating life of the bits. When a bit breaks or the cutting tip is used up or broken off, the bit is pried out of the support block and replaced by another bit. A tool of this general type for boring operations is described in the McKenry et al U.S. Pat. No. 3,720,273 granted Mar. 13, 1973.
In the prior art, as represented by the above-mentioned McKenry et al patent, the pick type cutters or bits have a cutter body or holder which comprises a cylindrical shank for insertion into the socket of the support block, a conical head adapted to receive the carbide cutting tip and a cylindrical section between the shank and the head. The shank is provided with an annular retaining groove and carries a keeper which retains the shank in the support block while allowing it to rotate. The cylindrical section between the head and the shank is provided with an annular extraction groove for receiving an extracting tool to pry the bit out of the block. The forward end or nose of the head is provided with a recess or socket for receiving the carbide insert or cutting tip.
The cutter body of the pick type cutter or bit of the prior art, as described in the McKenry et al patent, is made from steel stock by machining, as by a lathe or screw machine operation, to obtain the desired shape. This includes turning the retaining groove, extraction groove and the head to final shape as well as boring or drilling the socket for the cutting tip. The bit is completed by brazing the carbide cutting tip to the head. The bits made by this process are costly due to the several machining operations. Further, the resulting structure leaves much to be desired in respect to strength and other physical properties of the bit. Such bits have a relatively short life due to breakage, particularly at the braze between the carbide tip and the head. Also, the steel body lacks the desired toughness in the shank cylindrical section and head to suitably withstand the extremely high stresses imposed during tool operation.
An improved pick type bit is described in the Ewing U.S. Pat. No. 4,485,655 granted Dec. 4, 1984 and assigned to one of the assignees of this invention. As described in the Ewing patent, the entire cutter body is constructed of work-hardened steel. This is obtained by cold-heading the body to form an intermediate blank and then, after the cold-heading, the retaining groove and the extraction groove are roll-formed to final dimensions. The work-hardening results from the cold-heading and additionally work-hardening is obtained by the roll-forming of the grooves. Finally, the socket for carbide insert is formed by machining and the insert is brazed to the head. This process produces a steel body with superior toughness, relative to machined bodies, especially in the region of the retention groove and the extraction groove. Further, the cold-heading and roll-forming offer cost advantages relative to the screw machine operation described above.
It remains, however, that it is desired to produce the pick type bits at even lower cost and to obtain an improved structure which will result in prolonged life of the bits.
Accordingly, a general object of this invention is to provide a pick type bit which overcomes certain disadvantages of the prior art.